There exist many fields wherein accurate measurement of a time duration is critical to the success of a system analysis. In many instances, the time interval of measurement is determined by transmitting a relatively short pulse of energy (having a wide bandwidth) and precisely measuring the arrival time duration of the received return pulse. Typically, however, the return pulse is not identical to the transmitted pulse and in many instances the return pulse can be severely affected by the media through which the pulse travels. Typical examples of the use of a measured time interval are the radar and sonar environments wherein the time interval measures distance from the source to an object, for example, an airplane or the sea bottom. Another example of the use of time interval measurement is flow detection and measurement using ultrasonic signal energy, such as that described in Lynnworth, U.S. Pat. No. 3,575,050, issued Apr. 13, 1971, wherein a short pulse of ultrasonic energy is transmitted through a moving fluid in an upstream and a downstream direction. The time intervals of upstream and downstream travel provide measurement data useful in determining fluid flow.
In the ultrasonic flow measurement application in particular, the received pulse often represents the pulse as though it were transmitted through a narrow band filter. In clamp-on flowmeters, for liquids in steel pipe (when the acoustic impedance of the pipe exceeds that of the liquid by more than one order of magnitude), the pipe reverberations cause the received pulse to appear narrowband. At other times, electrical noise-rejection narrowband filters are used to improve the signal-to-noise ratio, and/or the transducers can be quarter-wave impedance matched into low impedance fluids, for example, petrochemical refining flare system headers, yielding a narrowband received signal. In each example, the time extent of the received pulse increases; and therefore, when accurate time durations are required, it is often difficult to exactly measure, consistently, when the pulse is received. In those instances where the time of receipt remains substantially constant from pulse transmittal to pulse transmittal, and/or when the pulse amplitude and shape remain substantially constant from pulse to pulse, relatively standard procedures are available for accurately determining the time when the pulse is received. Thus, for example, a typical approach is to measure the amplitude of the returning pulse; and, when that amplitude exceeds a fixed voltage threshold value, to set the time of receipt as the time of the next zero crossing of the pulse signal. This method is adequate in relatively noise free environments, or where the transmit time is relatively constant from measurement to measurement, and, under those circumstances, produces an accurate "relative" time duration. In the ultrasonic flow measurement system, it is the difference in transit time of the upstream and downstream pulse signals which is important and hence the arrival time, if determined in a consistent manner (even if the time measurement contains a constant error), is adequate for measuring the flow within the pipe.
In many flowmeter instances, however, there is significant noise on the received signal from, for example, interference within the pipe due to either turbulence or pipeline irregularities. In other instances, the transit time varies significantly due to time varying flows and turbulence of the flow. As a result, a typical zero crossing measurement based upon the amplitude threshold method described above, proves inadequate to the task of determining, with a high degree of accuracy, the pulse receive time for a narrow bandwidth pulse signal. In essence, the difficulty is determining the same zero crossing, for example the fifth, for each and every pulse signal received.
It is therefore an object of this invention to accurately measure the time of arrival of a narrow bandwidth pulse signal. Another object of the invention is accurately determining the arrival time of an ultrasonic pulse signal in a volumetric flow measuring environment. Further objects of the invention are a reliable, accurate, easily maintained intervalometer apparatus and method for accurately determining the arrival time of a pulse signal under conditions of varying flow rates and turbulence of the flow. Yet further objects of the invention are an intervalometer method and apparatus which is cost effective to build and easy to manufacture.